By Dan Falk with NBC News Tech and Science News&bullet;
last updated:
12/03/2019

Get ready for Pi Day! The annual celebration, held every March 14, is your chance to pay tribute to the most famous constant in math and physics: the number you get when you divide the circumference of a circle by its diameter. It's represented by the Greek letter "π" — in English, "pi."

Pi shows up in many formulas in math and physics. Its value is often rendered as 3.14 — hence the celebration on the 14th day of the third month of the year — but that's an approximation. The real value of pi is 3.1415926535… with the three dots signifying that the string of digits goes on forever.

In its endless stream of digits, pi is believed to contain the numbers 0 through 9 in every conceivable combination, forming every possible string of digits. Your phone number, social security number, ATM code and every other string of numbers you can imagine are in there somewhere. If you convert letters into numbers (for example, "c-a-b" could be "3-1-2"), then every essay you've ever written and every book you've ever read is in there — along with the complete works of Shakespeare.

"This hasn't been proven — but in theory, if you choose some string of a million digits, it's in there somewhere — and then it's in there again, infinitely often," says Patrick Ingram, a mathematician at York University in Toronto.

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You can search the first 200 million digits of pi at the Pi Search website, maintained by David Andersen, a professor of computer science at Carnegie Mellon University in Pittsburgh.

2. Pi is ancient.

Though it wasn't called pi until the 18th century, the numerical relationship between a circle's diameter and circumference has been pondered since antiquity.

In the second millennium B.C., the Babylonians used 25/8 for pi (equivalent to 3.125), while the Egyptians are believed to have used 256/81 (equivalent to 3.160).

In a pair of passages in the Bible (1 Kings 7:23 and 2 Chronicles 4:2), the altar in King Solomon's temple is said to be "10 cubits from brim to brim" while "30 cubits did compass it round about" —dimensions that seem to place the value of what we now call pi at 3.

The symbol π was first used to denote the circumference-to-diameter ratio in 1706 by Welsh mathematician William Jones. But it didn't catch on until Swiss mathematician Leonhard Euler adopted its use in the 1730s.

3. We've used computers to calculate pi to more than 22 trillion digits.

In 2016, a Swiss scientist, Peter Trueb, used a computer with 24 hard drives and a program called y-cruncher to calculate pi to more than 22 trillion digits — the current world record for the enumeration of pi. If you read one digit every second, it would take you just under 700,000 years to recite all those digits.

4. Humans have memorized vast stretches of pi.

At least since the 1970s, math geeks have competed informally to recite from memory as many digits of pi as possible. In 2015, Suresh Kumar Sharma, a former vegetable vendor from Jaipur, India (he's now a memory coach), set a world record when he successfully recited more than 17,000 digits of pi — a feat that took 17 hours to complete.

The U.S. record is held by Mark Umile of suburban Philadelphia, who in 2007 recited more than 15,000 digits of pi.

Umile said he committed the digits to memory by writing them down and then reading them aloud into a voice recorder — and listening to the recording again and again. He said it was gratifying to use his "sliver of Asperger's syndrome" — a condition he once viewed as debilitating — to mount "a successful endeavor that benefited my life and inspired others."

5. Pi has a bit part in many books and movies.

The protagonist of Carl Sagan's 1985 novel "Contact," astronomer Ellie Arroway, seeks evidence of extraterrestrial civilizations by listening for signals from space — and later looks for hidden patterns in the digits of pi (the latter plotline was cut from the 1997 movie version).

Fans of the original "Star Trek" series might remember "Wolf in the Fold," a 1967 episode in which Spock foils an evil computer by instructing it to compute pi to the last digit. And in Alfred Hitchcock's 1966 Cold War thriller "Torn Curtain," the secret network of agents that helps defectors escape the Soviet Bloc is codenamed "π."

6. Even rocket scientists only need a bit more than a dozen decimal places.

Though we know trillions of digits of pi, we don't really need them. Even the engineers at NASA round pi off to 15 decimal places when calculating interplanetary trajectories. In fact, if you were trying to calculate the size of the observable universe, using 39 digits of pi would give you an answer off by no more than the width of a hydrogen atom.

7. There are plenty of other reasons to mark March 14.

Aside from being Pi Day, March 14 is Albert Einstein's birthday. And physicist Stephen Hawking — considered by some to be Einstein's intellectual successor — died on March 14, 2018.